Process for the manufacture of maleic anhydride

ABSTRACT

A process for the manufacture of maleic anhydride by the oxidation of benzene with oxygen in the presence of a catalyst which has been previously activated and which comprises an inert support coated with a catalytically active mass containing by weight 10% to 20% of antimony trioxide, 40% to 70% of vanadium pentoxide, 20% to 50% of molybdenum trioxide, 0.7% to 1.5% of phosphorus pentoxide, 0.9% to 2.0% of sodium oxide and possibly another mineral oxide.

REFERENCE TO A RELATED APPLICATION

This is a continuation-in-part of copending application Ser. No. 525,599 filed Nov. 20, 1974 and now abandoned. The entire disclosure of said related application is relied on herein.

The present invention relates to a process for the manufacture of maleic anhydride.

Catalysts for the oxidation of benzene to maleic anhydride are already known. They are formed from mixtures of vanadium and molybdenum oxides to which may be added in smaller proportions other mineral oxides in order to improve the performance of the catalyst. These complex mixtures of oxides may be deposited on various supports, generally based on alumina or silica, or may be used as such in the form of powder, grains or agglomerates.

We have now found that excellent results are obtained in the oxidation of benzene to maleic anhydride if a catalyst is used which comprises an inert support coated with a catalytically active mass containing 10% to 20% by weight of antimony trioxide, 40% to 70% by weight of vanadium pentoxide 20% to 50% by weight of molybdenum trioxide, 0.7% to 1.5% of phosphorus pentoxide, 0.9% to 2.0% by weight of sodium oxide and possibly an other mineral oxide.

Such a catalyst, compared with the known catalysts, shows improved activity, selectivity and length of life. Industrially, this is manifested both by an increased yield of maleic anhydride with respect to the benzene used and by the possibility of operating with high concentrations of benzene in air, e.g. one mole of benzene per 10 to 16 moles of oxygen, during its passage over the catalyst. Further if the intrinsic qualities of the catalyst remain unchanged for many months, the production of maleic anhydride with relation to the volume of catalyst used is very high e.g. 120 to 150 g per hour for each liter of catalyst.

According to the present invention therefore a process for the manufacture of maleic anhydride by the oxidation of benzene with oxygen in the presence of a catalyst which has been previously activated is provided in which the catalyst comprises an inert support coated with a catalytically active mass containing, by weight 10% to 20% of antimony trioxide, 40% to 70% of vanadium pentoxide, 20% to 50% of molybdenum trioxide,, 0.7 to 1.5% of phosphorus pentoxide, 0.9 to 2.0% of sodium oxide and possibly another mineral oxide.

In addition to the antimony trioxide, vanadium pentoxide, molybdenum trioxide, phosphorus pentoxide and sodium oxide, the catalytically active mass may contain various oxides such as for example the oxides of nickel, calcium, iron, silver, strontium, uranium, tungsten, cobalt or boron.

In the catalysts according to the invention, the catalytically active mass preferably represents 10% to 20% of the total weight of the catalyst. The latter is preferably in the form of spherically or irregularly-shaped grains of which the average particle size is from 3 to 8 mm.

The inert support may for example comprise an aluminium and/or silicon derivative or a mixture of such derivatives. Examples of such inert supports are alumina, silica, aluminium silicate and silicon carbide.

The catalysts according to the invention may be prepared by known techniques. Thus, for example, the oxides of the catalytically active mass and/or the corresponding salts may be dissolved or suspended in water or in an organic solvent and projected on to the inert support placed in a coating apparatus maintained at a temperature sufficient to allow the evaporation of the water or the solvent as the solution or suspension is deposited on the support. Alternatively, the inert support and the liquid mixture may, for example be mixed in a heated coating drum and the mixture evaporated to dryness. The coated support is then calcined in the presence of a slight excess of air in the coating apparatus itself or in a muffle furnace at a temperature of 400° C. to 500° C. for at least 2 hours.

The catalysts must be kept out of contact with air and moisture until they are used. When they are used, the catalysts must be previously activated by heating in the presence of air at a temperature of from 300° C. to 600° C. for 6 to 24 hours.

In a preferred embodiment of the invention the oxidation of benzene is effected in a tube of inside diameter 21 mm, and maintained at a temperature of from 300° C. to 400° C. by an isothermal bath. The hot point of the catalyst is maintained at a temperature of from 400° C to 450° C., preferably at about 430° C. 10 to 16 moles of oxygen are used per mole of benzene, and the oxygen is preferably applied in the form of air at atmospheric pressure or under a slight superatmospheric pressure which may go up to 2 atmospheres.

The invention is illustrated by the following Examples. In these Examples the sodium oxide originates from the sodium phosphate.

EXAMPLE 1

800 g. of silicon carbide in spherical grains 6 mm in diameter obtained by fritting are introduced into a 2-liter coating apparatus. The grains are heated to about 180° C. and then a solution of 45.5 g. of ammonium paramolybdate, 84 g. of ammonium metavanadate, 8.4 g. of trisodium phosphate and 20 g. of antimony oxide in 690 g. of 33% concentrated hydrochloric acid, is sprayed on their surface in a period of about 1 hour. When the spraying is finished, the grains are heated to 410° C. and maintained at this temperature for 4 hours under a slight current of air. A catalyst having the following composition by weight is thus obtained:

Silicon carbide (as support): -- 86.38%

Molybdenum trioxide: -- 4.00%

Vanadium pentoxide: -- 7.07%

Phosphorus pentoxide: -- 0.17%

Antimony trioxide: -- 2.16%

Sodium oxide -- 0.22%

This catalyst is introduced into a vertical steel tube with an inside diameter of 21 mm, placed in a "salt bath" comprising a mixture of equal parts of potassium nitrate and sodium nitrite. This bath is stirred and raised to a temperature of 350° C. and to effect activation of the catalyst a stream of air heated to 150° C. is passed into the tube at 2000 g/hour and in a period of 10 hours the temperature of the salt bath is taken to 420° C. This temperature and the stream of air are then maintained for 12 hours.

After having lowered the temperature of the salt bath to 370° C. a mixture of 150 g/hour of benzene and 3300 g/hour of air is circulated in the tube. A production of 142 g/hour of maleic anhydride, i.e. 147 g/hour/liter of catalyst, is thus obtained at the outlet of the reaction tube. This production is still 130 g/hour after 3000 hours of continuous operation of the catalyst for the same consumption of benzene.

EXAMPLE 2

On operating as in Example 1 but with 1130 g. of pure alumina in spherical grains of average diameter from 5 to 6 mm and a solution of 51 g. of ammonium paramolybdate, 98 g. of ammonium metavanadate, 8.4. g. of trisodium phosphate and 15 g. of antimony oxide in 750 g. of 33% hydrochloric acid, a catalyst having the following composition by weight is obtained:

Alumina (as support): -- 89.23%

Molybdenum trioxide: -- 3.28%

Vanadium pentoxide: -- 6.03%

Phosphorus pentoxide: -- 0.12%

Antimony trioxide: -- 1.18%

Sodium oxide: -- 0.16%

This catalyst is activated in a reaction tube as in Example 1 with 2000 g/hour of air at 380° C. for 24 hours.

After having lowered the temperature of the salt bath to 360° C. a mixture of 135 g/hour of benzene and 3300 g/hour of air is circulated in the tube. 125 g/hour of maleic anhydride, i.e. 120 g/hour/liter of catalyst is thus obtained at the outlet of the reaction tube.

The production of maleic anhydride is still at 124 g/hour for the same quantity of benzene consumed after 3000 hours of continuous operation.

EXAMPLE 3

The operation is as in Example 2, but with a spraying solution prepared from 59 g. of ammonium paramolybdate, 132 g. of ammonium metavanadate, 10 g. of trisodium phosphate, 8 g. of cobalt nitrate, 25 g. of antimony trioxide and 750 g. of 33% hydrochloric acid.

A catalyst having the following composition by weight is obtained:

Alumina (support): -- 86.12%

Molybdenum trioxide: -- 3.66%

Vanadium pentoxide: -- 7.84%

Phosphorus pentoxide: -- 0.14%

Cobaltous oxide: -- 0.16%

Antimony trioxide: -- 1.90%

Sodium oxide: -- 0.18%

When a mixture of 158 g/hour of benzene in 3300 g. of air is circulated over this catalyst, under the same conditions as in Example 2, 144 g/hour of maleic anhydride are obtained corresponding to a production of 137 g/hour/liter of catalyst.

After 3000 hours operation under the same conditions, the production of maleic anhydride is still 139 g/hour.

EXAMPLE 4

A coating solution is prepared by dissolving 64.5 g. of ammonium paramolybdate, 149 g. of ammonium metavanadate and 28 g. of antimony trioxide in 800 g. of 33% hydrochloric acid, and adding to the mixture a solution of 90 ml. of 13% trisodium phosphate.

The final solution obtained is then introduced into a 2-liter coating vessel having a stirrer and containing 1200 g. of a mixture based on alumina (85%) and silica (13%) in irregular grains of dimensions from 4 to 6 mm. The mixture is heated for 2 hours at 90°-100° C. to dryness, then the coated grains are rapidly introduced into a muffle furnace previously raised to a temperature of 420° C. and in which a slight current of air of about 70 N liters/hour circulates. After 2 hours calcining the catalyst is withdrawn and is contingently stored in sealed bottles. Its composition by weight is as follows:

Support: -- 85.61%

Molybdenum trioxide: -- 3.75%

Vanadium pentoxide: -- 8.28%

Phosphorus pentoxide: -- 0.16%

Antimony trioxide: -- 2.00%

Sodium oxide -- 0.20%

This catalyst is placed in a reaction tube where it is activated under the conditions of Example 2. After the temperature of the salt bath has been lowered to 362° C. a mixture of 145 g/hour of benzene and 3300 g/hour of air is introduced at the catalyst entrance. 136 g/hour of maleic anhydride corresponding to 121 g/hour/liter of catalyst are obtained at the outlet of the reaction tube.

After 3000 hours of continuous operation with the same benzene feed, the production of maleic anhydride is 132 g/hour. 

We claim:
 1. A process for the manufacture of maleic anhydride by the oxidation of benzene with oxygen in the presence of a catalyst which has been previously activated and which comprises an inert support coated with a catalytically active mass containing, by weight, 10% to 20% of antimony trioxide, 40% to 70% of vanadium pentoxide, 20% to 50% of molybdenum trioxide, 0.7% to 1.5% of phosphorus pentoxide, 0.9% to 2.0% of sodium oxide and optionally another mineral oxide selected from the group consisting of, nickel, calcium, iron, silver, strontium, uranium, tungsten, cobalt, and boron oxides.
 2. A process according to claim 1 in which the catalytically active mass represents 10% to 20% of the total weight of the catalyst.
 3. A process according to claim 1 in which the catalyst is present in the form of spherically or irregularly-shaped grains having an average particle size of 3 to 8 mm.
 4. A process according to claim 1 in which the inert support comprises alumina, silica, silicon carbide, aluminium silicate or a mixture of these compounds.
 5. A process according to claim 1 in which the hot point of the catalyst is maintained at a temperature of from 400° C. to 450° C.
 6. A process according to claim 1 in which the hot point of the catalyst is maintained at a temperature of about 430° C.
 7. A process according to claim 1 in which 10 to 16 moles of oxygen are used per mole of benzene.
 8. A process according to claim 1 in which the oxygen is used in the form of air.
 9. A process according to claim 1 in which the oxygen is used in the form of air at a pressure of up to two atmospheres. 